test-writer

Test Writer Skill

Overview

Generate comprehensive, maintainable test suites. Focuses on correctness, isolation, and readability - tests that catch real bugs and survive refactoring.

Principles

1. One assertion concept per test - each test validates one specific behavior
2. Descriptive names - test_<unit>_<scenario>_<expected> format
3. Isolation - no shared mutable state between tests; mock external dependencies
4. Determinism - no flakiness from time, randomness, or network
5. Coverage - happy path + edge cases + error paths

Step-by-Step Process

Step 1: Analyse the Code Under Test

Identify:

• Inputs: parameters, types, valid ranges, optional vs required
• Outputs: return values, side effects (file writes, DB calls, HTTP requests)
• Dependencies: external systems to mock (DB, HTTP, clock, filesystem)
• Behaviors: branching logic, loops, error handling paths

Step 2: Define Test Cases

For each function/method, write test cases for:

Category	Examples
Happy path	Valid inputs → expected output
Boundary values	0, -1, max int, empty string, empty list
None / null	Missing optional fields, None arguments
Type errors	Wrong types where applicable
Domain errors	Negative price, future birth date, invalid email
External failure	DB down, HTTP 500, file not found

Step 3: Python / pytest

import pytest
from myapp.billing import calculate_discount

class TestCalculateDiscount:
    def test_gold_tier_applies_20_percent(self):
        assert calculate_discount(price=100.0, tier="gold") == 80.0

    def test_standard_tier_applies_no_discount(self):
        assert calculate_discount(price=100.0, tier="standard") == 100.0

    def test_zero_price_returns_zero(self):
        assert calculate_discount(price=0.0, tier="gold") == 0.0

    def test_negative_price_raises_value_error(self):
        with pytest.raises(ValueError, match="Price must be non-negative"):
            calculate_discount(price=-10.0, tier="gold")

    def test_unknown_tier_raises_value_error(self):
        with pytest.raises(ValueError, match="Unknown tier"):
            calculate_discount(price=100.0, tier="diamond")

    @pytest.mark.parametrize("tier,expected", [
        ("gold", 80.0),
        ("silver", 90.0),
        ("bronze", 95.0),
    ])
    def test_tier_discounts_parametrized(self, tier, expected):
        assert calculate_discount(price=100.0, tier=tier) == expected